Summer Howard, 19, and Alecia Wilkins, 18, make a poster for Summer’s sister Rebecca Sedwick. Polk County Sheriff Grady Judd said that Sedwick jumped to her death on Sept. 9 at an old cement business in Lakeland, Fla.

TAMPA, Fla. — For nearly a year, as many as 15 girls ganged up on 12-year-old Rebecca Ann Sedwick and picked on her, authorities say, bombarding her with online messages such as “You should die” and “Why don’t you go kill yourself.”

Rebecca couldn’t take it anymore.

She changed one of her online screen names to “That Dead Girl.” She messaged a boy in North Carolina: “I’m jumping.” And then, on Monday, the Lakeland girl went to an abandoned concrete plant, climbed a tower and hurled herself to her death.

Authorities have seized computers and cellphones from some of the girls as they decide whether to bring charges in what appeared to be the nation’s latest deadly cyberbullying case.

The bullying started over a “boyfriend issue” last year at Crystal Lake Middle School, Sheriff Grady Judd said. But he gave no details. Police said Rebecca was suspended at one point for fighting with a girl who used to be her friend.

Rebecca had been “absolutely terrorized” by the other girls, Judd said. He said detectives found some of her diaries at her home, and she talked of how depressed she was about the situation.

“Her writings would break your heart,” he said.

The case has illustrated, once more, the ways in which youngsters are using the Internet to torment others.

“There is a lot of digital drama. Middle-school kids are horrible to each other, especially girls,” said Perry Aftab, a New Jersey-based lawyer and expert on cyberbullying.

Last December, Rebecca was hospitalized for three days after cutting her wrists because of what she said was bullying, according to the sheriff. Later, after Rebecca complained that she had been pushed in the hallway and that another girl wanted to fight her, Rebecca’s mother began home-schooling her in Lakeland, a city of about 100,000 midway between Tampa and Orlando, Judd said.

This fall, Rebecca started at a new school, Lawton Chiles Middle Academy, and loved it, Judd said. But the bullying continued online.

“She put on a perfect, happy face. She never told me,” Rebecca’s mother, Tricia Norman, told the Lakeland Ledger. “I never had a clue. I mean, she told me last year when she was being bullied, but not this year, and I have no idea why.”

After Rebecca’s suicide, police looked at her computer and found search queries such as “what is overweight for a 13-year-old girl,” “how to get blades out of razors,” and “how many over-the-counter drugs do you take to die.” One of her screensavers also showed Rebecca with her head resting on a railroad track.

Police said that she had met the North Carolina boy at an airport and that they had remained friends online. The 12-year-old boy didn’t tell anyone about the “I’m jumping, I can’t take it anymore” message he received from her on Monday morning, shortly before her suicide, authorities said.

Detectives said the other girls’ parents have been cooperative.

Florida has a bullying law, but it leaves punishment to schools, not police. Legal experts said it is difficult to bring charges against someone accused of driving a person to suicide.

“We’ve had so many suicides that are related to digital harassment. But we also have free-speech laws in this country,” Aftab said.

In a review of news articles, The Associated Press found about a dozen suicides in the U.S. since October 2010 that were attributed at least in part to cyberbullying. Aftab said she believes the real number is at least twice that.

In 2006, 13-year-old Megan Meier hanged herself in California after she was dumped online by a fictitious teenage boy created in part by an adult neighbor, Lori Drew, authorities said. A jury found Drew guilty of three federal misdemeanors, but a judge threw out the verdicts and acquitted her.

Florida’s law, the Jeffrey Johnston Stand Up for All Students Act, was named after a teenager who killed himself after being harassed by classmates. The law was amended July 1 to cover cyberbullying.

David Tirella, a Florida attorney who lobbied for the law and has handled dozens of cyberbullying cases, said law enforcement can also seek more traditional charges.

“The truth is, even without these school bullying laws, there’s battery, there’s stalking,” he said.